Titration curves and indicatorsEdexcel A-Level Chemistry: Revision notes

Section 1

Strong acid with strong base

For 25.0 cm³ of 0.100 mol dm⁻³ HCl with 0.100 mol dm⁻³ NaOH, the pH starts at 1.0, rises slowly, then climbs vertically from about pH 3 to pH 11 around the equivalence volume (25.0 cm³), and levels off near 13. The equivalence point is at pH 7, because NaCl does not affect the pH. After 30.0 cm³, excess OH⁻ = 5.0 × 10⁻⁴ mol in 55.0 cm³ gives $[OH^-]$ = 9.09 × 10⁻³ mol dm⁻³ and pH = 11.96.

Key termsequivalence pointvertical section

Common mistake

Dividing the excess alkali by the original 25.0 cm³. Use the total volume of the mixture.

Section 2

Weak acid with strong base

The curve starts at a higher pH (2.88 for 0.100 mol dm⁻³ ethanoic acid), rises quickly, then flattens in a buffer region and has a shorter vertical section, about pH 7.8 to 9.7, centred above 7. The equivalence point is alkaline (about pH 8.7) because the salt contains $CH_3COO^-$ , a weak base: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻. Suitable indicator: phenolphthalein.

Key termsbuffer regionsalt hydrolysis

Section 3

Strong acid with weak base, and weak with weak

Strong acid with weak base (HCl with NH₃): the curve starts at pH 1, with a vertical section of about pH 4 to 6, and the equivalence point is acidic (about pH 5.3) because NH₄⁺ is a weak acid. Suitable indicator: methyl orange.

Weak acid with weak base (ethanoic acid with ammonia): the pH rises gradually with no vertical section, so no indicator gives a sharp end point. Use a pH meter or conductivity.

Key termsmethyl orange

Exam tip

Quote the equivalence pH and the steep range when you justify an indicator.

Section 4

Choosing an indicator

An indicator is a weak acid HIn with a different colour from In⁻. Its colour changes over about ±1 pH unit around its pKIn. For a sharp end point its range must lie within the vertical section of the curve.

Indicator	Range
methyl orange	3.2–4.4
bromothymol blue	6.0–7.6
phenolphthalein	8.2–10.0

Strong–strong: any; weak acid–strong base: phenolphthalein; strong acid–weak base: methyl orange.

Key termsindicator

Common mistake

Saying an indicator changes colour at pH 7. Each has its own range.

Section 5

Buffer action and Ka from the curve

In the gently sloping section of a weak acid–strong base curve, HA and A⁻ are both present in large amounts. Added OH⁻ reacts with HA (HA + OH⁻ → A⁻ + H₂O), and the ratio $[HA]/[A^-]$ barely changes, so the pH changes little: this is buffer action.

At half the equivalence volume, $[HA] = [A^-]$ , so $K_a = [H^+]$ and pKₐ = pH. For HX, pH 3.80 at 12.5 cm³ gives $K_a = 1.58 \times 10^{-4}$ mol dm⁻³.

Key termsbuffer actionhalf-equivalence

Section 6

Enthalpy of neutralisation for strong and weak acids

For a strong acid with a strong base, the reaction is H⁺ + OH⁻ → H₂O, with ΔH ≈ −57 kJ mol⁻¹. A weak acid is only partly dissociated, so some energy released is used to ionise the acid (endothermic) as the equilibrium shifts. Its enthalpy change of neutralisation is therefore less exothermic (ethanoic acid ≈ −55 kJ mol⁻¹).

Calculation: $q = mc\Delta T$ , then ΔH = −q ÷ n(H₂O).

Key termsenthalpy change of neutralisation

Common mistake

Using the volume of one solution for m. The temperature rise applies to the total mass of the mixture.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Titration curves and indicators

A student titrates 25.0 cm³ of 0.100 mol dm⁻³ hydrochloric acid with 0.100 mol dm⁻³ sodium hydroxide solution from a burette, recording the pH with a calibrated meter at 298 K, where $K_w$ = 1.00 × 10⁻¹⁴ mol² dm⁻⁶.
Describe the change in pH on adding alkali close to the equivalence point and explain why it happens.2 marks
A student titrates 25.0 cm³ of 0.100 mol dm⁻³ ethanoic acid (pKₐ = 4.76) with 0.100 mol dm⁻³ sodium hydroxide solution at 298 K. The pH at the equivalence point is about 8.7. The indicators available are methyl orange (colour change over pH 3.2–4.4), bromothymol blue (pH 6.0–7.6) and phenolphthalein (pH 8.2–10.0).
Explain why methyl orange is not suitable for this titration.2 marks
A student titrates 25.0 cm³ of 0.100 mol dm⁻³ of an unknown weak monobasic acid, HX, with 0.100 mol dm⁻³ sodium hydroxide solution, using a pH meter. After adding 12.5 cm³ of sodium hydroxide solution the pH is 3.80, and the equivalence point is reached after 25.0 cm³.
Determine $K_a$ for the acid HX. Explain your method.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).